1. Field of the Invention
This invention relates to a method for the production of ethylene glycol. Particularly this invention pertains, in a composite process for obtaining ethylene oxide by catalytic gas phase oxidation of ethylene and causing the ethylene oxide to react with water thereby obtaining ethylene glycol, to a method for the production of ethylene glycol by advantageously utilizing in the composite process mentioned above the vapor generated in the multi-effect evaporator used in the step for concentrating the produced aqueous ethylene glycol solution.
2. Description of the Related Art
Ethylene glycol is usually produced by the reaction of ethylene oxide with water. Then, the ethylene oxide is produced nowaday by the catalytic gas phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst. The process for the production of ethylene oxide is roughly as follows.
The reaction gas containing ethylene oxide formed by the catalytic gas phase oxidation of ethylene with molecular oxygen-containing gas on the silver catalyst is introduced to an ethylene oxide absorber and brought into contact with an absorption liquid having water as a main component to recover the aqueous ethylene oxide solution and then forwarded to an ethylene oxide stripper so as to cause stripping of the ethylene oxide from the aqueous solution by heating the bottom portion of the ethylene oxide stripper with steam, the aqueous solution containing substantially no ethylene oxide and obtained from the bottom portion of the ethylene oxide stripper is cyclically used as the absorption liquid, The stripped products such as the ethylene oxide obtained from the top of the ethylene oxide stripper, water, carbon dioxide, inert gas (such as nitrogen, argon, methane, and ethane), such low boiling impurities as formaldehyde, and high boiling impurities as acetaldehyde and acetic acid are forwarded through the dehydration step, the light end separation step, and the heavy end separation step to obtain purified ethylene oxide. Portion of the gas containing the unreacted ethylene, by-produced carbon dioxide, and inert gases (such as nitrogen, argon, methane, and ethane) may be circulated to the ethylene oxidation step. Normally, it is partially separated and introduced to the carbon dioxide absorber so that carbon dioxide may be selectively absorbed therein and the absorbed solution may be treated to recover carbon dioxide therefrom by stripping.
The aqueous solution containing monoethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and polyethylene glyol which are obtained by the reaction of purified or crude ethylene oxide consequently formed with water is deprived of the water by vaporization as in a multi-effect evaporator. The concentration consequently obtained is dehydrated to a high degree and further purified sequentially in a monoethylene glycol distillation column, a diethylene glycol distillation column, and a triethylene glycol distillation column to obtain purified ethylene glycols. Incidentally, in the ethylene oxide absorber even in the process for the production of ethylene oxide, the reaction occurs between water with ethylene oxide and forms ethylene glycol.
The absorbing solution is partly separated and similarly concentrated in the multi-effect evaporator and/or the dehydration distillation column to obtain the various ethylene glycol products(Refer, for example, to xe2x80x9cChemical Processxe2x80x94Fundamentals to Development of Techniquexe2x80x94xe2x80x9d compiled by Society of Chemical Engineering and published by Tokyo Kagaku Dojin on Mar. 25, 1998, pages 121-128.).
The production of ethylene glycol from ethylene via ethylene oxide as described above entails various operations such carbon dioxide stripping operation, ethylene oxide stripping operation, dehydrating operation, light end separating operation, an ethylene oxide rectifying operation, by-produced ethylene glycol concentrating and dehydrating operation, and further mono-, di-, and triethylene glycol rectificating operations. Since these operations consume large amount of heat, they are under obligation to control supply of the heat volumes efficiently.
An object of this invention, therefore, is to provide for a composite process which subjects ethylene to catalytic gas-phase oxidization and causes the resultant ethylene oxide to react with water to produce ethylene glycol a method for the production of ethylene glycol which comprises advantageously utilizing for the composite process the steam generated at a multi-effect evaporator used at the step for dehydrating the aqueous ethylene glycol solution obtained by the composite process.
We, after pursuing a diligent study in search of a solution for the problems mentioned above, have conceived an idea of advantageously utitilizing the energy of the steam generated from a multi-effect evaporator serving to evaporate and concentrate the aqueous ethylene glycol solution obtained by the reaction of ethylene oxide with water. We have found that the problems mentioned above are consequently solved. This invention has been perfected as a result.
The object of this invention mentioned above is accomplished by the following item (1) and (2).
(1) A method for the production of ethylene glycol which in producing by the catalytic gas phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst to obtain ethylene oxide and causing the resultant ethylene oxide to react with water thereby producing an aqueous ethylene glycol solution and subjecting this aqueous solution to a concentrating operation with a multi-effect evaporator and dehydrating this aqueous solution and producing ethylene glycol, which comprises using the vapor generated in the multi-effect evaporator mentioned above as the source for heating at least one of the following steps (A)-(H):
(A) a step of introducing an ethylene oxide-containing gas formed by the reaction of catalytic gas phase oxidation to an ethylene oxide absorber, causing the gas to contact an aqueous medium absorption solution and form ethylene oxide-containing bottoms therein, introducing the bottom to an ethylene oxide stripper, and separating ethylene oxide by heating the bottoms of the stripper,
(B) a step of circulating portion of the gas from the top of the ethylene oxide absorber to the ethylene oxidation step and introducing the remainder thereof to a carbon dioxide absorber and allowing it to contact with an alkali absorption solution to obtain a carbon dioxide-containing bottom, and introducing the bottoms to the carbon dioxide stripper, and heating the bottoms of the stripper thereby separating carbon dioxide,
(C) a step of introducing an aqueous ethylene oxide solution obtained by concentrating the gas from the top of the ethylene oxide stripper to the ethylene oxide dehydration column and heating the bottoms of the dehydration column thereby separating light end components such as ethylene oxide,
(D) a step of introducing the ethylene oxide-containing fraction obtained by condensing the gas from the top of the dehydration column to the light end separation column, heating the bottoms of this separation column thereby separating the light weight component, and obtaining crude ethylene oxide as the bottoms,
(E) a step of introducing the crude ethylene oxide to the ethylene oxide rectifying column and heating the bottoms of the rectifying column thereby obtaining purified ethylene oxide from the top of the rectifying column,
(F) a step of extracting portion of the absorption solution obtained through the bottom of the ethylene oxide stripper, introducing it to the by-produced ethylene glycol concentration column and heating the bottoms of the concentration column thereby effecting dehydration and concentration,
(G) a step of introducing an aqueous ethylene glycol solution obtained at the multi-effect evaporator and concentrated therein to the ethylene glycol dehydration column, heating the bottoms of the dehydration column thereby effecting substantial separation of the water through the top of the column, and
(H) a step of introducing the solution of the ethylene glycol dehydration column substantially deprived of water to the monoethylene glycol distillation column bottoms, heating the bottoms of the distillation column thereby separating and obtaining monoethylene glycol from the top of the column.
(2) A method according to claim (1), wherein the number of multi-effect evaporators to be used is at least three and the steam to be utilized as the heating source has pressure in the range of xe2x88x920.08 to 1.2 MPa (Gauge).
It has been demonstrated that by partly removing the steam generated in the top portion of the multi-effect evaporator and utilizing the removed steam as the source for heating the other steps, it is made possible to recover the thermal energy possessed by the steam, attain effective utilization of energy and, with a smaller consumption of the energy than the total amount of energy used at the other steps and in the other multi-effect evaporator, fulfill the distillation at the other steps and the evaporation and concentration in the multi-effect evaporator.